Describing Holmgang as an "invincibility" doesn't quite grok with me. Yes, it prevents you from dying for the duration, but it's not *true* invincibility, like Hallowed Ground, which is what it always gets compared to. The survivability value of Holmgang, even if the bind were removed, is *very* different from what Holmgang provides.
I do understand that it's necessary to group abilities of that level together; I just don't like the category being referred to as "invincibility CDs". A better term, as I see it, would be "uber-CD" or "super CD", since that's really what they are. I've sometimes heard of them referred to as "survivability nukes" ("nuke" referring to a single massive attack on a long CD, which we don't really have in ARR).
As a design point, I *do* think that it's appropriate for all tanks to have a *single* ability that exists in that category. It doesn't necessarily have to copy HG or Holmgang, but I think it behooves the devs to make sure that all tanks have some massive last-resort CD. Even if PLD and WAR didn't start with one, I still think it would be apt to include them, just because it's an appropriate functionality for a tank to have. Unlike DPS, tanks and healers are both classes that often have compressed need for functionality. Burst DPS doesn't really matter in the course of PvE content, but burst survivability and burst healing are very important. Healers have abilities like Lustrate and Benediction to fulfill their need to adapt to the situations that require *incredible* compression and Divine Seal, Sacred Soil, Rouse, and Fey Illumination for times when functional compression is needed, but not on the same scale; tanks have their normal CDs for basic compression, but HG and Holmgang are needed for those "shit hits the fan" moments when you need to have absolute survivability for a short window. Building tanks without last resort CDs would be like building healers without Benediction/Lustrate: they *can* get away with it, but it's a niche functionality that deserves at least a single tool to cover.